[ros2] segfault during shutdown

[clydemcqueen]

In Foxy (though it looks like it's been in the code forever):

rqt_image_view segfaults during shutdown if there is an active subscription.

I'm using printf debugging and saw that ImageView::callbackImage is often called after ImageView::shutdownPlugin. There might be a better way to fix this, but it was easy to fix by adding bool shutting_down_ to guard the callback. Happy to provide a PR.

[MatthijsBurgh]

I am running on ros1, but I experience the same issue. But this also happens in other CPP rqt plugins. So it isn't related to rqt_image_view. I opened an issue, ros-visualization/qt_gui_core#258

For me applying #58 to the ros1 code didn't solve it. Which is in line with my other observations.